Today, video stream transmission is rapidly becoming an integral part of access network traffic. In video stream transmission, both broadcast transmission and multicast transmission play important roles. In a nutshell, a broadcast transmits information to all of a device's potential destinations simultaneously while a multicast can transmit information to just a subgroup of destinations simultaneously. There are several techniques that deal with broadcast for TDM access networks such as BPON/GPON/EPON. These techniques include using a sub-carrier multiplexing technique or a frequency division multiplexing (FDM) technique. The sub-carrier multiplexing technique multiplexes digital base-band and RF video signals in frequency domain and then modulates the mixed signal onto a single wavelength. The frequency division multiplexing (FDM) technique (a CWDM-based approach) uses a separate wavelength for video.
However, traditionally, broadcasting a data or video stream to all subscribers at once has been difficult in a Wavelength-Division-Multiplexed Passive Optical Network (WDM-PON) architecture. The reason for this difficulty is that the output ports of the wavelength selective devices of the Optical Distribution Network (ODN) only pass a specific wavelength channel on each specific output port destined to a subscriber, as shown in FIG. 1. Hence, there is no one common wavelength that reaches all customers.
FIG. 2 shows one way of overcoming this limitation in the WDM-PON architecture. In the implementation shown in FIG. 2, a bypass WDM coupler is used to separate the video signal (which is on a separate wavelength) before the AWG demux, and then the video wavelength is split using a 1×16 power splitter and added to each user's downstream data wavelength. The extra bypass splitter and other WDM couplers required in this scheme are an overhead which can be avoided by intelligent use of components. Note that the number of WDM couplers required scales with the number of subscribers (16, 32, 64 etc.) and the number of fusion splices scale as 3× the number of subscribers.
FIG. 3 shows another scheme that allows video broadcast in the WDM-PON architecture. This scheme uses an N×N AWG, a laser operating at 1530 nm in the OLT for downstream broadcast video, and WDM lasers in the 1550 nm band for downstream WDM data. The ONUs use LEDs in the 1310 nm band for upstream data. At the remote node, the downstream WDM channels are first passed through a 1310 nm/1550 nm coupler for separating the upstream and downstream channels. The 1530 nm downstream video is then separated from the data and split into 15 components using a 1×15 splitter. These 15 video streams are distributed to the 15 ONUs via a 16×16 AWG router. The last port of the AWG router demultiplexes the 15 WDM data streams sending each to its corresponding ONU. The downstream data and video can then be detected using a PIN photodiode at the ONU. The N×N Remote Node can be further optimized by incorporating the 1×15 splitter into the AWG for a single device implementation.
FIGS. 4A and 4B show yet another scheme for broadcasting video in a WDM-PON architecture. This scheme uses broadcast and selection. FIG. 4A shows the network for this scheme and FIG. 4B shows the structure of ONU pairs. The downstream link from the OLT to the remote node is a simple downstream CWDM link which is then split and broadcast to each of the N ONUs. At the ONUs, the downstream data is separated from the broadcast video using a band filter. Then, the video is passed through a coupler to get the received video stream. The data stream is passed through a 3-port TFF for separating the receive data and adding the transmit data to the stream. The data stream is then multiplexed again with the video stream to form a protection path for the neighboring ONU. Although this scheme gives a simplified network structure and provides a protection path for the ONUs, the extra fiber required for connecting the ONU pairs and complexity of each ONU may make this impracticable.
The scheme shown in FIG. 5 uses a broadband light source for broadcast downstream. The WDM data and broadcast video are sent in a TDM fashion using a fast optical switch on the OLT. The video broadcast stream is transmitted by using a broadband light source with an external modulator. The network structure including the Remote node does not change for accommodating the broadband video transmission. The ONU can use the same receiver for receiving both the data and video to time de-multiplex the two.
Each of the above methods has its disadvantage(s). For example, the N×N AWG scheme shown in FIG. 3 requires several components leading to excess loss for the video signal (2 WDM couplers(2 dB)+1×16 splitter (13 dB)+N×N AWG(6 dB)=21 dB). Integrating the splitter with the N×N AWG can be tricky due to the requirement for the latter to be an athermal device, which in turn requires special packaging techniques. If not integrated, the device can be bulky due to several splices/connectors between the couplers, the splitter and the AWG. The broadcast and selection scheme shown in FIGS. 4A and 4B increases the complexity of the ONU and requires extra fiber layout. The TDM scheme shown in FIG. 5 reduces the data bandwidth because of time multiplexing and the noise performance will degrade at higher data rates due to the modulation of a broadband source.
A device and method that allows broadcasting in a WDM-PON architecture without the above disadvantages is desired.